The atmospheric pressure interface (API) of a mass spectrometer is used to transfer ions from a region at atmospheric pressure into other regions at reduced pressures. It allows the development and use of a variety of ionization sources at atmospheric pressure for mass spectrometry, including electrospray ionization (ESI) (Fenn, J. B.; Mann, M.; Meng, C. K.; Wong, S. F.; Whitehouse, C. M. Science 1989, 246, 64-71; Yamashita, M.; Fenn, J. B. J. Phys. Chem. 1984, 88, 4451-4459), atmospheric pressure ionization (APCI) (Carroll, D. I.; Dzidic, I.; Stillwell, R. N.; Haegele, K. D.; Horning, E. C. Anal. Chem. 1975, 47, 2369-2373), and atmospheric pressure matrix assisted laser desorption ionization (AP-MALDI), (Laiko, V. V.; Baldwin, M. A.; Burlingame, A. L. Anal. Chem. 2000, 72, 652-657; Tanaka, K.; Waki, H.; Ido, Y.; Akita, S.; Yoshida, Y.; Yoshida, T.; Matsuo, T. Rapid Commun. Mass Spectrom. 1988, 2, 151-153) etc. An API not only allows the coupling of a mass spectrometer with various sample separation and sample pretreatment methods, such as liquid chromatograph, but also enables ambient preparation and treatment of ions using a variety of desirable conditions, such as the thermal production of the ions, (Chen, H.; Ouyang, Z.; Cooks, R. G. Angewandte Chemie, International Edition 2006, 45, 3656-3660; Takats, Z.; Cooks, R. G. Chemical Communications (Cambridge, United Kingdom) 2004, 444-445) ion-ion reactions (Loo, R. R. O.; Udseth, H. R.; Smith, R. D. Journal of the American Society for Mass Spectrometry 1992, 3, 695-705) or ion fragmentation, (Chen, H.; Eberlin, L. S.; Cooks, R. G. Journal of the American Chemical Society 2007, 129, 5880-5886) before sending them into vacuum for mass analysis. Without an API, it is also not possible to take advantage of the recent development of a new category of direct ambient ionization/sampling methods, including desorption electrospray ionization (DESI) (Takats, Z.; Wiseman, J. M.; Gologan, B.; Cooks, R. G. Science 2004, 306, 471-473), direct analysis in real time (DART) (Cody, R. B.; Laramee, J. A.; Durst, H. D. Anal. Chem. 2005, 77, 2297-2302), Atmospheric Pressure Dielectric Barrier Discharge Ionization (DBDI), and electrospray-assisted laser desoption/ionization (ELDI) (Shiea, J.; Huang, M. Z.; Hsu, H. J.; Lee, C. Y.; Yuan, C. H.; Beech, I.; Sunner, J. Rapid Commun. Mass Spectrom. 2005, 19, 3701-3704).
Since the ESI source was first successfully demonstrated for mass spectrometry (Yamashita, M.; Fenn, J. B. J. Phys. Chem. 1984, 88, 4451-4459), the configuration of API used for ESI was widely adopted and has not changed significantly. Nowadays a typical API has a constantly open channel involving a series of differential pumping stages with a capillary or a thin hole of small ID to allow ions to be transferred into the first stage and a skimmer for access to the second stage. A rough pump is usually used to pump the first region to about 1 torr and multiple turbomolecular pumps or a single pump with split flow used for pumping the subsequent regions with a base pressure in the final stage used for the mass analysis, which is usually 10−5 torr or below. Ion optical systems, including static electric lenses and RF guides, are also used to preserve the ion current while the neutrals are pumped away. To maximize the number of ions transferred into the final region for mass analysis, large pumping capacities are always desirable so that larger orifices can be used to pass ions from region to region. As an example, a Finnigan LTQ (Thermo Fisher Scientific, Inc., San Jose, Calif.) ion trap mass spectormeter has two 30 m3/hr rough pumps for the first stage and a 400 l/s turbomolecular pump with two drag pumping stages for the next 3 stages. The highest loss in ion transfer occur at the first stage and the second stage, corresponding to a 2 orders and a 1 order of magnitude, respectively, which results in an overall efficiency lower than 0.1% for the ion transfer through an API. When an attempt is made to implement this kind of API on a portable instrument, the ion transfer efficiency is further reduced by the fact that much lower pumping capacity must be used to achieve the desirable weight and power consumption of the instruments. A recently developed Mini 10 handheld rectilinear ion trap mass spectrometer weighs only 10 kg and has miniature rough and turbo pumps of only 0.3 m3/hr and 11 l/s, respectively. (Gao, L.; Song, Q.; Patterson, G. E.; Cooks, R. G.; Ouyang, Z. Anal. Chem. 2006, 78, 5994-6002)
Many efforts have been made to increase the ion transfer efficiency in laboratory scale mass spectrometers. The ion transfer through the second stage has been successfully improved by a factor of ten by replacing the skimmer with an ion funnel. (Shaffer, S. A.; Tang, K. Q.; Anderson, G. A.; Prior, D. C.; Udseth, H. R.; Smith, R. D. Rapid Communications in Mass Spectrometry 1997, 11, 1813-1817) Air-dynamic ion focusing devices (Zhou, L.; Yue, B.; Dearden, D. V.; Lee, E. D.; Rockwook, A. L.; Lee, M. L. Anal. Chem. 2003, 75, 5978-5983; Hawkridge, A. M.; Zhou, L.; Lee, M. L.; Muddiman, D. C. Analytical Chemistry 2004, 76, 4118-4122) have been employed in front of API's of mass spectrometers. Though the efficiency of API itself was not improved, the ultimate ion current reaching the mass analyzer was significance increased. However, the possibility of arcing inside the vacuum increases at high pressure, which results in high noise and short lifetime of the electron multiplier and power supplies.
There is a need for atmospheric interfaces that increase ion transfer efficiency to a mass spectrometer.